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Convert MGED from Tk to Qt

BRL-CAD's graphical applications (MGED and Archer in particular) make use of the Tcl/Tk toolkit to describe their interfaces. Tk has some limitations when it comes to complex cross-platform application development, and we are looking to migrate our graphical interface codes from Tk to the Qt toolkit.

There are two major "directions" that can be attempted:

Re-create MGED as a Qt application. MGED is our current "production" geometry editing and viewing interface.

Most likely recreating one of these interfaces in its entirity is well beyond the scope of GSoC (although we certainly won't complain if you manage it!) A better strategy is to identify and tackle one aspect of the problem with an eye towards creating a Qt widget that can be reused in subsequent applications. If you complete that task, you can build from there.

Easily the most important "missing" component at this point is the 3D interactive viewing widget - the centeral widget in Archer's interface and the "3D display" window in MGED. A good starting point would be to create a "Qt viwer widget" that can load and display a libdm 3D context. You will need to handle Qt's OpenGL widget and its interactions with libdm/libfb, set up key bindings using our existing Tk bindings as a guide, and ensure that multile views can work successfully (the quad view mode).

Another major and important piece of the interface is the tree widget (Archer's is the example to study, and a read-only version has been implemented in the experimental qged code - the challenge will be to first get it updating in response to geometry changes in the database. After that there are any number of additional features needed...)

If you're less comfortable with graphical programming, we're also constructing a custom CAD terminal widget. We have a basic implementation in our experimental qged GUI, but it needs work - text selection doesn't span command outputs, we need a way to auto-hide long outputs that aren't the current output in the history and allow them to expand/contract via some user mouse interaction, we need an "in-process" graphic for commands that have launched but haven't yet completed, ideally with a way to abort them as well (this will require some delving into QThreads and launching libged commands in separate threads, as well as a limited control-the-view terminal mode while some potentially database altering command is running.)

Anyone wanting to tackle this project should have a good familiarity with things like the Qt event and signals/slots systems as well as the generic widget systems (most of the pre-packaged Qt widgets aren't going to be quite what we need, which usually means creating custom widets with the more general parent classes.) See the qged sources below for an idea of the approach.

Proposals here should be detailed and demonstrate a firm grasp of the issues. Qt 5.3 or higher should be used (that is the assumption made in the existing code, and if you require a newer Qt for some feature or issue that should be fine - supporting older Qt releases is not something we're going to spend effort on at this early stage.